The main hypothesis is based on the premise that cotrimoxazole is more effective than IPT-SP for placental parasitaemia. This might be due to the higher plasma concentration of cotrimoxazole attained with daily doses. If this hypothesis is proven, cotrimoxazole could be recommended as prophylaxis for HIV-positive pregnant women, whatever their CD4+ cell count. In this study, the investigators will also test the hypothesis that the strains of Plasmodium falciparum isolated from HIV-positive pregnant women express more dhfr and dhps resistance markers.